Using resources provided by a Phase I SBIR, ViroLogic has successfully developed a rapid, single-cycle replication assay for the assessment of HIV replication capacity (RC) that is reproducible, accurate, and precise. A full laboratory validation has been completed and the assay has achieved CLIA-CAP certification. Replication capacity measurements determined using this assay correlate with in vivo "fitness" competition assays, and basic investigations to probe the enzymatic, genotypic, and phenotypic correlates of alterations in replication capacity are ongoing. Importantly, numerous clinical studies now suggest that replication capacity as measured by the PhenoSense assay correlates inversely with CD4 counts and directly with viral load measurements, such that a low RC value is associated with CD4 preservation and reduced levels of HIV RNA, even in the face of ongoing viral replication. Furthermore, replication capacity has recently been shown to be a predictor of clinical progression to AIDS that is independent of HIV RNA, CD4 count, and age. However, the clinical utility of replication capacity remains unclear. The observations made in multiple small clinical studies now need to be confirmed in large, well-characterized studies using cohorts with virological, immunological, and clinical outcome measures and longitudinal banked samples available for testing. The overarching goals of the project are to confirm the correlations with CD4 and HIV RNA that have been suggested by the small scale studies performed to date, to determine whether RC can be used in clinical settings to predict progression to AIDS, to define the natural history of RC in untreated patients, to understand whether correlations that exist (if any) between RC and clinical outcomes can be generalized to a variety of patient subtypes, to learn whether absolute RC values or the change in RC is a better predictor of clinical outcome, and to gain a better understanding of how to properly interpret RC values for optimum clinical benefit. We will accomplish these specific aims by means of careful analysis of replication capacity in multiple large cohorts that offer a variety of patient subtypes and span the breadth of treatment experience from naTve to heavily treated. We propose to collaborate with a stellar group of investigators who specialize in the epidemiology of HIV/AIDS with biostatistical consultation from acknowledged experts. We anticipate performing approximately 2500 replication capacity determinations over a three year period. [unreadable] [unreadable]